In the manufacture of glassware, it is common practice to apply two coatings to the outside surface of the glassware. One of these coatings is called a "hot-end" coating and the other is called a "cold-end" coating.
Hot-end coatings are applied to the glassware at the hot end of the annealing lehr. Hot-end coatings are used to provide increased chemical resistance, increased strength and durability, and increased resistance to scratches and abrasions.
Cold-end coatings are applied to the glassware after annealing and after the glassware has cooled from annealing temperatures. Cold-end coatings are used to provide a lubricating coating and thereby permit smooth flow of the containers through high speed inspection and filling lines.
Hot-end coatings consist of a metallic oxide which is applied as the vapor of a metallic compound. Common materials for use as hot-end coatings include titanium tetrachloride and tin tetrachloride.
When the metallic compound that has been deposited on the glassware is subjected to the pyrolyzing temperature of the compound, which is commonly above 315 degrees Celcius (600 degrees Farenheit), the metallic compound decomposes to form a thin metallic oxide coating that is permanently bonded to the surface of the glassware.
Since hot-end coatings are applied after removing the glassware from the molding machine, and prior to the glassware entering the annealing lehr, the temperature of the glassware is near 537 degrees Celcius (1000 degrees Farenheit), and thus far above the minimum pyrolyzing temperature; so no additional heat is required for the hot-end coating process.
While hot-end coatings impart desirable chemical and mechanical properties to glassware, it is important that they not be applied to the threaded portion of glass containers that use metal lids.
A metallic oxide coating on the threaded portion of the glass container increases the friction between the container and the metallic lid; and so accidentally depositing the coating on the threaded portion can result in an excessively high torque requirement for removal of the lid.
In addition, if a metallic oxide coating is applied to the threaded portion of glass containers, galvanic corrosion may occur between the metal lid and the metallic oxide coating in the presence of normal moisture in the air. The reason for this is the difference in electrical potentials between the metallic oxide coating and the metal of the lid or its electroplated coating.
Unfortunately, it is impossible during the application of the metallic vapor to see whether or not the vapor is being applied accidentally to the threaded portion of the glassware. Further, after pyrolyzing and cooling, it is often very difficult to determine without a chemical or optical test whether or not some of the threaded portion has received the metallic oxide coating.
Therefore, it is important to provide apparatus or process in which the vapor of the metallic compound is directed onto the glassware with accuracy; so that accidental coating of the threaded portion of the glass container is prevented.
One possible solution to the problem is to mechanically protect a portion of the glassware from receiving the vapor of the metallic compound. In U.S. Pat. No. 3,615,327, McLary disclosed apparatus for suspending glassware by the portion that is to be protected from the spray, and for carrying the glassware through the spray hood by the suspended portion.
However, because of the great variety of sizes of glass containers, both in diameter and height, automating a device such as has been disclosed by McLary is unduly complex and expensive.